Modem refractory castables are designed to have operating properties to meet a wide range of environmental and temperature conditions. In addition to operating properties, controlled properties such as setting times, hardening kinetics and rheology, are also important. In this respect, most modem refractory castables use admixtures in order to modify or control such properties. Admixtures basically interact with the hydraulic binder system, i.e., the calcium aluminate cement, during hydration, thereby influencing properties such as the workability, setting time and early strength of the refractory castable. As with all hydraulic binders, the chemical and mineral composition of the calcium aluminate cement generally determine the rheology, reactivity and refractoriness of the system. This does not mean, however, that the setting time and flow behavior of a refractory material depend, for a given water cement ratio, only on the cement used. The constituents other than the cement, such as for example, admixtures greatly influence the setting time and flow behavior of a refractory system.
For refractory compositions, the major admixtures used are commonly referred to as "accelerators," "retarders" and "plasticizers" (water-reducing agents). Accelerators generally accelerate or quicken the setting time of a refractory, whereas retarders generally slow down the setting time of the refractory material. Plasticizers are used to reduce the amount of water used in the refractory, and greatly affect the flow characteristics of the refractory material. A combination of retarders and accelerators is possible, and in most instances, recommended. In this respect, the two effects do not necessarily cancel each other out, but can affect the refractory material at different stages of hydration to produce a working time sufficient to allow forming of the material followed by a quick setting of the same. Admixtures interact not only with the calcium aluminate cement, but also with fine reactive materials and even with the aggregate present in the refractory system. Thus, working time, setting time and hardening can be controlled by a retarder/accelerator mixture to provide a more user-friendly control of the hydration kinetics.
Fine reactive fillers such as reactive alumina or silica fume, that are not conventionally considered as "admixtures," also affect the rheology, setting time, and even the physical properties of the refractory system. In this respect, some reactive fillers show effects that are very similar to the effects of classical admixtures as a result of their interaction with cement and the admixtures.
The present invention provides a refractory system including a reactive metakaolin-based additive or filler which may be used in place of a portion of calcium aluminate cement, which metakaolin-based filler enhances the cast properties of the refractory system, as well as properties relating to the rheology, setting times and hardening kinetics of the refractory system.